This invention relates in general to the filtration of dust laden gases and more particularly to a bag type dust filter having an improved system for cleaning f the filter bags.
In bag type dust filtration equipment, dust laden air is passed through a bank of filter bags which remove the dust. In this particular type filter, the bags are normally suspended from a tube sheet located between an underlying dirty air plenum and an overlying clean air plenum. The dust laden air is drawn into the dirty air plenum and through the filter bags into the clean air plenum from which the filtered air is discharged. The dust which is removed from the air collects on the outside surfaces of the filter bags and must be periodically dislodged to maintain the bags in condition to effectively perform their filtering function. When the filter housing is cylindrical, cleaning of the filters is often performed by a rotating arm which applies pulses of reverse flowing air through the bags to remove the accumulated dust. This type of cleaning system is generally shown in U.S. Pat. No. 4,157,899 to Wheaton.
Cleaning of the filter bags with a radial cleaning arm presents a number of difficulties which are caused in part by the geometry. The cleaning arm normally has a single row of discharge nozzles for applying the air pulses. However, it is not desirable to arrange the filter bags in rows extending along radial lines on the tube sheet because this type of arrangement spaces the bags in the outer rings so far apart that effective use of the tube sheet area is not made. If the filter bags are arranged to achieve efficient filtration, they are not located on common radials and the cleaning arm is not able to clean all of the filters in a uniform manner. If air is applied to the cleaning arm while some of its discharge nozzles are misaligned with the underlying bags, at least some of the cleaning air is wasted because it is directed against the surface of the tube sheet where it does not good. When the air is discharged randomly from the arm, the effectiveness of the cleaning air application to any one bag is purely a matter of chance, and the uniformity of the cleaning operation suffers accordingly.
Flow losses have also detracted from the cleaning efficiency. The cleaning air is typically accumulated in a tank that may be located some distance from the cleaning arm. Because of the distance the compressed air must travel between the tank and arm, there are significant flow losses. The distance between the accumulator tank and the cleaning arm also increases the response time of the system, resulting in a delay between opening of the main valve and application of the cleaning air pulses to the filter bags.